The taxman handed out a record one and a half million fines to late filers this year, with penalties up 56% in just five years, according to a Freedom of Information request.
The request by the Telegraph revealed that the number of fines issued by HM Revenue & Customs' automatic penalty system jumped 8% since last year.
Law firm McGrigors warned that many taxpayers are being unfairly charged, with HMRC "churning out fines all too indiscriminately".
The amount payable has also changed from a fixed £100 penalty to a maximum of £1500, depending on the amount of tax owed.
The next tax deadline is October 31, by which date all those who are planning to file their tax return on paper, rather than using the internet, should file their returns.
If you send a paper return after this date you will be fined, an HMRC spokesman warned.
Jason Collins, from McGrigors, said those who have a "reasonable excuse" for missing the tax return deadline shoudl appeal any fine, and said that there have been several appeal cases suggesting that HMRC has been "too draconian".
An HMRC spokesperson said: "We want tax returns back not penalties, so nobody will receive a penalty where they file a tax return by the deadline or have a reasonable excuse for failing to do so.
"Penalties exist to encourage people to file on time."
Based on ONS data
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